The present invention broadly relates to a coil bobbin and an exciting coil assembly. More particularly, the invention is concerned with a coil bobbin suited for making an exciting coil assembly to be used in an electromagnetic clutch as well as with such an exciting coil assembly.
In explaining a conventional coil bobbin and exciting coil assembly, general construction of an electromagnetic clutch is described at first. Generally, an electromagnetic clutch is constructed as illustrated in FIG. 8. Designed for use in a paper feeding mechanism of a copying machine, for instance, the electromagnetic clutch illustrated comprises a field housing 32 configured in a bottomed double cylindrical body furnished with a cylindrical mounting sleeve 31 which is passed through the center of the field housing 32. A cylindrical shaft 33 is rotatably fitted inside the mounting sleeve 31 of the field housing 32. Inside the field housing 32, a ring-shaped exciting coil assembly 34 is mounted around the mounting sleeve 31.
A rotor 36 having slits 35 located at positions facing the exciting coil assembly 34 is fixedly mounted on the shaft 33 while an armature 37 is rotatably mounted on the shaft 33 at a position facing the rotor 36. Further, a driving force transmission mechanism 38 including a gear is rotatably mounted on the shaft 33 at a position facing the armature 37, which is integrally fixed to the driving force transmission mechanism 38 with a ring-shaped flat spring 39 interposed in between.
The shaft 33 has retaining ring mounting grooves 40 and 41 formed close to its both ends, and retaining rings 42 and 43 are fitted in the retaining ring mounting grooves 40 and 41, respectively, to prevent the individual components mentioned above from coming off the shaft 33. Among the aforementioned components, at least the field housing 32, shaft 33, rotor 36 and armature 37 is constructed with a magnetic material, such as a ferrous material.
In the electromagnetic clutch thus constructed, the driving force transmission mechanism 38 driven by an unillustrated driving source rotates around the shaft 33 together with the armature 37. When the exciting coil assembly 34 is energized, magnetic flux is generated by the exciting coil assembly 34 through a magnetic circuit formed by the field housing 32, shaft 33 and rotor 36. Leakage flux occurring in the slits 35 of the rotor 36 produces an attractive force and, therefore, the rotor 36 attracts the armature 37, overwhelming an elastic force of the flat spring 39. As a result, the rotor 36 rotates together with the driving force transmission mechanism 38, and the shaft 33 to which the rotor 36 is fixed also rotates.
When the exciting coil assembly 34 is de-energized, the attractive force between the rotor 36 and armature 37 is lost since the magnetic flux disappears. In consequence, the armature 37 is separated from the rotor 36 and pulled back toward the driving force transmission mechanism 38 due to the elastic force of the flat spring 39.
The exciting coil assembly 34 of the above-described electromagnetic clutch is constructed as illustrated in FIGS. 9 and 10, where FIG. 9 is an exploded perspective view of the exciting coil assembly 34, and FIG. 10 is a side view of a coil unit 34a of the exciting coil assembly 34. As shown in FIG. 9, the exciting coil assembly 34 comprises the coil unit 34a and a cover unit 34b which is mounted over the coil unit 34a. The coil unit 34a includes a cylindrical body 42' made of an insulating material, which has a through hole 41' in the middle and a pair of ring-shaped flanges 43' and 44 at both ends, and a coil 45 wound around a coil winding portion of the cylindrical body 42' formed between its flanges 43' and 44. Both ends 45a and 45b of the coil 45 are individually passed through a pair of slits 46 and 47 formed in the flange 43' and soldered to ends of a pair of lead wires 48 and 49 on an outside surface of the flange 43'.
The pair of lead wires 48 and 49 to which the ends 45a and 45b of the coil 45 are connected are run through a lead wire guide 50 projectingly formed on the periphery of the flange 43' to external circuitry. On the outside surface of the flange 43', there are provided an inner ring-shaped projection 51 and an outer ring-shaped projection 52, forming a recessed portion 53 therebetween to allow the pair of lead wires 48 and 49 to be arranged without projecting from the outside surface of the flange 43'. The pair of lead wires 48 and 49 accommodated in the recessed portion 53 are laid along guideways provided by a plurality of guiding projections 54, 55 and 56 formed in the recessed portion 53 as well as the inner ring-shaped projection 51 and outer ring-shaped projection 52 and led out through the lead wire guide 50.
The cover unit 34b is a one-piece formed component including a coil cover 57 which, made of a film-like insulating material, wraps around the coil 45 and a flange cover 58 which covers the outside surface of the flange 43'. The inside diameter of the coil cover 57 is same as the outside diameter of the flanges 43' and 44 of the cylindrical body 42'. The coil cover 57 has at its top a straight cut 59 and a cutout 60 into which the lead wire guide 50 of the flange 43' is fitted. The flange cover 58 has a through hole 61 at a middle position corresponding to the through hole 41' of the cylindrical body 42' and a projection 62 located at the position of the cutout 60 in the coil cover 57 for covering the outside surface of the lead wire guide 50.
When the cover unit 34b is mounted on the coil unit 34a, the coil cover 57 encloses the coil 45 and the flange cover 58 covers the outside surface of the flange 43' to complete the exciting coil assembly 34. The following procedure is used when fitting the cover unit 34b on the coil unit 34a. With the straight cut 59 in the coil cover 57 spread out upward, the cover unit 34b is placed in contact with the coil unit 34a on the outside surface of its flange 43'. As the cover unit 34b is forced against the coil unit 34a, the lead wire guide 50 of the flange 43' slips into the cutout 60 and the cover unit 34b is properly fitted on the coil unit 34 a. Then, an adhesive tape is attached over the straight cut 59 to prevent it from opening and the cover unit 34b from coming off the coil unit 34a.
The reasons why the exciting coil assembly 34 is constructed by covering the coil unit 34a with the cover unit 34b are as follows. This construction facilitates soldering work since both ends 45a and 45b of the coil 45 can be soldered to the pair of lead wires 48 and 49 on the exposed outside of the flange 43'. The coil unit 34a is covered with the cover unit 34b for preventing a short circuit of the coil 45 which may otherwise occur should the exposed solder joints between both ends 45a and 45b of the coil 45 and the pair of lead wires 48 and 49 come into contact with adjacent components when assembled into the electromagnetic clutch. The above-described construction of the exciting coil assembly 34 provides ease of soldering and good electric insulation performance of the joints between both ends 45a and 45b of the coil 45 and the pair of lead wires 48 and 49.
The conventional exciting coil assembly 34 is constructed by fitting the coil unit 34a into the cover unit 34b as seen above. Although the conventional exciting coil assembly 34 has the aforementioned advantages, it entails rather complicated management of constituent components, and assembling of the coil unit 34a and cover unit 34b is essential in manufacturing stages, making it difficult to streamline its assembly work.
The coil 45 completely fits into the coil winding portion of the cylindrical body 42' without producing any outward projection according to the above-described construction of the conventional exciting coil assembly 34. Therefore, no problem will arise concerning electric insulation performance even when the coil cover 57 is eliminated. The flange cover 58 is, however, essential for ensuring electric insulation performance because the solder joints between both ends 45a and 45b of the coil 45 and the pair of lead wires 48 and 49 are exposed on the outside surface of the flange 43' and could potentially stick out of the flange 43'. It is therefore impossible to eliminate the cover unit 34b in the above-described construction of the conventional exciting coil assembly 34.